1. Field of the Invention
The present invention relates to a method of making a polycrystalline silicon for a thin film transistor, a mask pattern used in the same and a method of making a flat panel display device by the same. More particularly, the invention relates to a method of making a polycrystalline silicon for a thin film transistor to reduce non-uniformity of luminance generated in a display device using a polycrystalline silicon fabricated according to sequential lateral solidification (SLS) crystallization technology, a mask pattern used in the method of making a polycrystalline silicon for a thin film transistor, and a method of making a flat panel display device by the method of making a polycrystalline silicon for thin film transistor.
2. Description of the Related Technology
Ordinarily, sequential lateral solidification (SLS) crystallization is a method for crystallizing grain silicon by directing a laser beam onto overlapping regions of an amorphous silicon layer two or more times so as to anneal the silicon, and thereby laterally grow silicon crystals, or grains. Polycrystalline silicon grains fabricated by the SLS crystallization method are each formed in a columnar shape having a long length in one direction, and grain boundaries are formed between adjacent grains.
It has been reported that polycrystalline or single crystal large silicon grains can be formed on a substrate by SLS crystallization technology, and characteristics similar to characteristics of a thin film transistor (TFT) fabricated of single crystal silicon are obtained when fabricating a TFT using the polycrystalline or single crystal large silicon grains.
FIG. 1A, FIG. 1B and FIG. 1C are drawings showing an ordinary SLS crystallizing method.
The SLS crystallizing method uses an annealing process, wherein amorphous silicon is melted in the laser beam transmission region when the laser beam is directed onto an amorphous silicon thin film layer through a mask having a laser beam transmission region and a laser beam non-transmission region as illustrated in FIG. 1A.
Crystallization starts at an interface between solid and molten silicon when cooling. A temperature gradient is formed in which the temperature gradually decreases in a direction from the interface to the molten silicon.
Therefore, referring to FIG. 1B, a polycrystalline silicon thin film layer is formed having grains of a long columnar shape oriented in a specific direction. Since the heat flows in a direction from the interface of the mask to the central part of a molten silicon layer, the polycrystalline silicon grains are laterally grown until the molten silicon layer is completely solidified. Thus, grains are grown from an amorphous silicon layer that is not melted such that grain boundaries are formed between adjacent grains. A grain boundary formed in a direction perpendicular to the direction of grain growth is a “primary grain boundary,” as shown in FIG. 1B.
As illustrated in FIG. 1C, amorphous and crystalline silicon are melted by directing a laser beam onto the amorphous and polycrystalline silicon layer through a mask moved from one position to a next. The mask is moved through a predetermined sequence of positions and the laser beam is directed on to the silicon at each position. As shown in FIG. 1C, during this process, the mask is moved in such a way that a portion of the silicon to be melted and crystallized in the mask's second position overlaps the silicon melted and crystallized in the mask's first position. This is done so that the silicon melted in the mask's second position will crystallize according to the crystal structure of the silicon crystallized in the mask's first position. That is, the silicon crystallized in the mask's first position becomes a seed for the silicon melted in the mask's second position. This method has the advantageous effect of creating grains of increased length. Thus, the size of grains in a SLS crystallization method is based on the size of laser transmission regions on the mask and the number of repeated laser treatments.
FIG. 2 is a drawing schematically illustrating a mask pattern used in a method of making a polycrystalline silicon thin film by a conventional two shot type SLS crystallization technology. FIG. 3 is a drawing illustrating energy density in a laser beam used when fabricating polycrystalline silicon using the mask pattern of FIG. 2. Also shown is polycrystalline silicon fabricated according to the energy density.
Because the energy density in a laser beam may not be uniform, the energy density of a central part may be higher than that of the edge as illustrated in FIG. 3. As a result crystallinity of the crystallized polycrystalline silicon varies and the characteristics of TFT vary accordingly.
Therefore, non-uniformity of luminance is generated on a display if the energy density of the laser is not uniform when crystallizing amorphous silicon by an SLS crystallization technology.
FIG. 4A is a plan figure showing a mask used in a conventional laser shot mixing method of crystallizing silicon. As shown, on the right half, the mask has a first area comprising a single transmission region, and on the left half the mask has a second area comprising two transmission regions. The laser shot mixing method is used to crystallize a non-crystallized region by crystallizing a first portion of the region when the mask is in a first position. The mask is then moved to a second position such that the left half of the mask then occupies the space previously occupied by the right half of the mask. A second portion of the region is then crystallized with the mask in the second position. Accordingly, the second portion of the region overlaps the first portion of the region in a direction normal to the direction of mask movement. FIG. 4B and FIG. 4C are plan figures showing the mask and the silicon at progressive stages. FIG. 4B shows the mask and the silicon after the silicon has been crystallized with the mask at the first position and after the mask has been moved to the second position. Note that the silicon has been crystallized in locations corresponding to the transmission regions of the mask when the mask was in the first position. Note also the overlap of the transmission regions when the mask is in the second position with the silicon crystallized when the mask was in the first position. FIG. 4C shows the silicon after a first crystallization when the mask was in the first position and after a second crystallization when the mask was in a second position. Note that the center region of the silicon, which was exposed to the laser through transmission regions in the mask while the mask was in both the first and the second positions, is completely crystallized. That is, from top to bottom there are no gaps in the crystallized region. As the process continues, each successive regions receives two crystallizing laser exposures, and is completely crystallized. Also note, however, that the areas between regions are not exposed to laser treatment, and therefore are not crystallized.
FIG. 5 is a photograph showing stripe marks formed in the polycrystalline silicon fabricated by the SLS crystallization method with the conventional laser shot mixing technology described above with reference to FIG. 4A, FIG. 4B and FIG. 4C. The visual difference seen is an indication that the crystallization process is not entirely uniform. Such non-uniformity results in non-uniform transistor conductance properties and non-uniform luminance, and is, therefore, undesirable.
In addition, the laser shot mixing process suffers from a disadvantage that process time is increased because four shot processes are used to form at least one crystal. The mask of FIG. 4A has another disadvantage related to processing time. The mask of FIG. 4A is configured such that crystals grow substantially perpendicular to the direction of movement. Therefore to generate crystals with grain boundaries advantageously oriented, the mask is moved in a horizontal direction across the display with the display oriented as when viewed. Because the horizontal direction is longer than the vertical direction in the display, the movement is in the longer direction, and therefore an excessive number of mask movements are necessary.
In PCT international patent No. WO 97/45827 and U.S. Pat. No. 6,322,625, technologies for converting amorphous silicon into polycrystalline silicon and crystallizing only a selected region of the amorphous silicon with a SLS crystallization method after depositing amorphous silicon on a substrate are disclosed.
Furthermore, a crystallization method resulting in improved TFT characteristics is disclosed in U.S. Pat. No. 6,177,301. According to U.S. Pat. No. 6,177,301, the barrier effect grain boundaries have on charge carriers is minimized when the channel direction is parallel to the direction of grains grown by the SLS crystallizing method when fabricating a TFT for an LCD device including drivers and pixels. Conversely, when the grain boundaries are perpendicular to the direction of the channel, the grain boundaries act as traps and result in a TFT of inferior characteristics.
In circuits which include transistors which are perpendicular to one another, the crystal growth can be induced in a direction oriented ideally about 45 degrees with respect to the orientation of all transistors. This results in the grain boundary effects being generally matching among the transistors, and avoidance of much of the trap effect caused by grain boundaries being perpendicular to the channels. In some applications this advantageous effect may be achieved with angles of orientation from about 30 degrees to about 60 degrees. However, like the laser shot mixing technology described above with reference to FIG. 4A, FIG. 4B and FIG. 4C, the method of U.S. Pat. No. 6,177,301 also suffers from non-uniformity of the polycrystalline silicon grains according to non-uniformity of laser energy density. This results in similar non-uniformity of luminance in the display. Another disadvantage common to the method described above with reference to FIG. 4A, FIG. 4B and FIG. 4C, and the method of U.S. Pat. No. 6,177,301 is the uncrystallized areas between the crystallized regions, as shown in FIG. 4C.
A method for complete crystallization is described in Korean Patent Laid-open Publication No. 2002-93194 in which laser beam patterns are formed in a triangle shape (), and crystallization proceeds while moving the triangle shaped () laser beam patterns widthwise.